Physicists create vortex knot -- akin to 'tying a smoke ring'

Physicists from the University of Chicago have worked out how to create a knotted vortex in fluid -- a
task they describe as being akin to tying a smoke ring into a
knot. Vortex knots have been known to be theoretically
possible, but scientists have never known how to create them in the
laboratory.

Vortices are naturally occurring in nature -- most often seen in
air and water but also plasma, such as on the Sun. They are also
observed in smoke rings and form around aeroplane wings. Knotted
and linked vortices (think two links in a chain) have been
conjectured to exist in neutron stars, in the plasma on the Sun and
in fluids.

The team created the vortex knots by creating 3D-printed
knotted, wing-like structures [see video above]. These hydrofoils
(wings that go into water) were attached to a frame and lowered
into water to be coated with bubbles released from a grid at the
bottom of the tank. The bubbles get caught on the 3D printed
structure. That structure is then rapidly accelerated down into the
water before it stops dead, releasing the bubbles in whatever shape
the structure was printed. The team experimented with simple loops
as well as more complex knotted structures. They then filmed the
development and dissipation of the vortices at a very high frame
rate.

The team found that vortex knots behaved in an unexpected way.
They theorised that they should be stable phenomena, but they
weren't. According to postdoctoral scientist Dustin Kleckner,
"They seem to break up in a particular way. They stretch
themselves, which is weird behaviour."

Two linked or knotted loops tend to elongate and then start to
circulate in opposite directions. They then move towards each other
and collide, causing parts of the vortices to destroy other parts
and change their configuration so that they become unlinked or
unknotted.

The knots may be useful for understanding complicated behaviour
of electrically charged gas in plasma flows and the energy
transport of complex flows in fluids.

Edited by Ian Steadman

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